The present invention relates to a driving device, especially for a timepiece. The invention includes a step-by-step or stepping motor comprising a stator and a rotor. The stator comprises a coil having only one winding arranged around a core which constitutes or establishes a magnetic circuit, the core having a yoke surrounding a free space, and the free space having substantially the shape of a cylinder of revolution. The rotor is constituted by a permanent magnet having at least one pair of magnetic poles diametrically opposed and is rotatably mounted in the free space. The invention further includes means for controlling the motor able to deliver to the coil pulses of current.
This driving device is mainly intended to drive, through the intermediary of a gearing, the hands of an electronic watch having an analog display.
In most electromechanical watches which are presently known, one uses a stepping motor receiving driving pulses at a fixed and precise frequency. The pulses are produced from a control circuit driven by a quartz oscillator followed by a frequency divider. The motor is arranged in such a way as to be able to rotate only in one sense or direction corresponding, obviously, to the normal sense or clockwise rotation of the hands of the watch.
There are, however, some cases where a motor able to rotate in two senses, clockwise and counterclockwise, is suitable, especially when one desires to provide the watch with a system of correction, automatic or not, of its time cations, or with a system permitting to pass from one time-zone to another without losing the exact time.
Motors able to rotate clockwise and counterclockwise have been developed, as well as suitable control circuits.
Some of these motors are provided with two windings into which the control circuit send pulses of current, the pulses being alternated according to a determined order which determines the sense of rotation.
Other motors are provided with only one winding, into which the control circuit sends alternating pulses of current. When the rotor is in one of its positions of balance, there being two such positions of balance which are determined, for instance, by the shape of the stator, a pulse in one sense or a first polarity, arbitrarily designated as being of positive polarity, produces a rotation of the rotor a half a revolution in the clockwise direction for instance. The rotor then reaches its other position of balance, in which to produce a new rotation of half a revolution in the clockwise direction, it is necessary to send a pulse of an opposite sense or polarity, designated as being of negative polarity. From the first position of balance, a negative polarity pulse produces a rotation of the rotor a half a revolution in the counterclockwise direction, rotation which can be continued from the other position of balance by the sending of a positive polarity pulse.
This arrangement or device shows or exhibits the drawback that, if after a shock for instance, the rotor of the motor has made an untimely half a revolution, or has not reacted to the sending of a driving pulse, its sense or direction of rotation is reversed, even when not desired, since the succeeding pulses continue in the pre-determined order, but the position of the rotor is not what it should be.
The positions of balance of the rotor are generally determined by an asymetric shape which is given to or formed in the stator of the motor. This asymetry produces a difference in the driving torque according to the direction of rotation. That is a drawback, since it produces a reduction in the efficiency of the motor which is very disagreeable in an electronic watch where the consumption must be as low as possible.
On the other hand, in most of the electronic watches having an analog display which are presently known, a stepping motor is used which receives pairs of pulses of current at a fixed and precise frequency, as already mentioned. At the end of each of these pairs of pulses, or when the motor is at rest, one takes the necessary steps, presently, so that the coil of the stator of the motor be is short-circuited by the of control circuit of the motor so as to obtain a substantial braking of the rotor, so that the rotor will be rapidly immobilized at the end of a step. This braking is obtained by an action of the induced current circulating in the coil immediately after the end of the pulses of current.
One knows also the control device of a stepping motor rotating clockwise and counterclockwise, each step being 180.degree., such as is disclosed in U.S. Pat. No. 4,066,947. This control device is also arranged in such a way as to short-circuit the coil of the motor when there is no pulse of current.
The drawback of these arrangements is due to the fact that a non-negligible part of the electric energy transmitted to the motor is absorbed uselessly by the damping of the movement of the rotor when there is no pulse of current.
The purpose of the present invention is to remove the mentioned drawbacks.
This purpose is reached owing to the means as claimed.